In personal digital assistants including cell phones and in various instruments, input keyboards for personal computers, remote controllers, etc., film type lightguide sheets for efficiently illuminating the input key parts are being used in recent years from the standpoint of an improvement in design or an improvement in design attractiveness. LEDs are used as a light source therefor, and such a lightguide sheet is configured so that the light emitted by the LEDs disposed at an end of the lightguide sheet is transmitted through the sheet and diffused to render the input key part evenly luminous, thereby causing the whole surface to evenly shine at even brightness without reducing the luminance of the LEDs. Furthermore, since use of a lightguide sheet makes it possible to reduce the number of LED chips, a reduction in electric-power consumption and a reduction in cost are rendered possible (patent documents 1 to 3).
Meanwhile, with the trend toward performance advancement in information processing appliances and portable digital assistants, the wiring within the appliances has come to be required to attain a higher speed and to have a higher density. Optical wiring, which is capable of high-speed transmission and reduced in electromagnetic noise as compared with the conventional electrical wiring, tends to be introduced in order to meet such requirements, and film type optical waveguides are being developed enthusiastically. In order for a film type optical waveguide to be used for these purposes, the optical waveguide must be capable of transmitting light to a more distant place while minimizing light loss.
It is known that the effect of confining light is higher when the difference in refractive index between the core, which is high in refractive index, and the clad, which is low in refractive index, is larger (non-patent document 1). There is hence a desire for a transparent core material having a higher refractive index.
Known as materials having a high refractive index are, for example, cured resins represented by optical lenses and organic-inorganic hybrid materials in which inorganic particles having a high refractive index, such as, for example, a metal oxide, have been evenly dispersed on a nanometer level.
For example, patent document 4 discloses a composite material constituted of tetragonal zirconia having a size of the order of nanometer and a resin.
Patent document 5 describes an inorganic-organic composite material which includes a resin matrix and fine inorganic particles evenly dispersed therein and which is most suitable for use as optical parts having improved properties with respect to moisture resistance, transparency, and refractive index, in particular, as lenses. The patent document shows a technique in which a dispersant having a number-average molecular weight of 50-10,000 and containing carboxyl groups in a specific proportion is used in order to evenly disperse the fine inorganic particles in the resin. Furthermore, incorporation of a thermoplastic resin produced using a thiol derivative having a carboxyl group at an end is also described therein.